Another Herpesvirus

H for Human Cytomegalovirus!

If you are a regular reader of my blog, then you know how much I like herpesviruses (studying them, that is, not having them).

It occurred to me that I had never written about the herpesvirus that I spent the majority of my post graduate school career studying!

Human cytomegalovirus, or HCMV, has the largest genome of the human herpesviruses, but it is nowhere near the size of Pandoravirus.  It encodes about 200 genes.  In addition to genes that make proteins, HCMV also expresses small RNAs that can turn genes off; either its own genes, or the genes of the person it is infecting.

The lesser known herpesvirus. I am always surprised that no one has heard about HCMV (I mean no one outside of my sphere of herpes virologists), because there is a good chance that you have it!  Depending on where you grow up and live, 40-80% of the population is infected with HCMV.  And as I have said before, herpesviruses are friends for life; they hide inside your body for as long as you are alive.

The reason you don’t hear about it?  It’s pretty benign for most people. If you are healthy, you will probably have no symptoms when infected with HCMV.  Sometimes you might get a sore throat, or some people might develop a mononucleosis (the mono we are more familiar with is caused by another herpesvirus called Epstein Barr Virus).  Whether you have symptoms or not, after the first infection, you will randomly shed infectious virus in your bodily fluids throughout your life. This includes, saliva, urine, blood, semen and breast milk.  While not extremely contagious, HCMV is often passed around between family members and in day care centers.

The reasons we study HCMV.  I studied HCMV because I was interested in the interaction between viruses and the immune system.  HCMV is a master manipulator of the human immune system.  It encodes many genes dedicated to shutting down or changing the immune system so the virus can spend its life inside you without your body getting rid of it.  Yet it does so in a subtle way that doesn’t (usually) cause sickness.

People always used to ask me if I was making a vaccine for HCMV.  The honest answer – not really.  Maybe eventually something I was studying would turn out to give us some information that could be useful for a vaccine, but that was a (really) long shot.

I studied it because it was fascinating, but fascinating doesn’t get you funding.  The reason why the government is interested in HCMV research is because there are 3 groups of people that have trouble with HCMV:  immunocompromised people, developing fetuses and the elderly.  As one of my friends used to say, “HCMV likes to kick people when they are down.” Right now I am only going to talk about the first two groups.

There is an ever-evolving dance going on between HCMV and the immune system.  HCMV finds ways to avert the immune system then the immune system fights back and keeps the virus in check.  But if the immune system becomes compromised, then HCMV can swirl out of control and cause disease in any number of organs.  It causes blindness, pneumonia and liver failure in AIDS patients, cancer patients and organ transplant recipients taking immunosuppressant drugs.

HCMV is also the leading infectious cause of birth defects.  The virus passes from the mother to the fetus where in utero infections can have severe consequences on the developing child.  In the direst circumstances, HCMV can cause death of the fetus. More often, HCMV will cause handicaps.  Five out of every 1000 live births have HCMV.  5% of these will have lasting effects including deafness, blindness, seizures and/or mental retardation.

To put this in perspective, children born with complications due to HCMV are more common than those born with Down Syndrome and far more common than those children born with HIV, as shown in this graph from the Centers for Disease Control and Prevention:

HCMV treatment.  HCMV can be treated with anti-viral drugs.  But these can be toxic and the virus can become resistant to the treatment.  Treatment is usually reserved for those with life-threatening illness or in danger of losing their sight.  Vaccine research has been going on for decades, but an effective vaccine remains elusive (very frustrating for those doing the research).

It’s difficult to eradicate something that has been with us since before we were humans!